1. Field of the Invention
The invention relates to information recording systems. More specifically, the present invention relates to a recording system having a novel liquid bearing interface between a transducer and a storage medium that provides improved reliability and performance over the currently-used air bearing. The invention applies particularly to information recording systems which use a disk for information storage and an air- or oil-borne transducer head for placing information on, and retrieving information from, the disk.
2. Background of the Art
The tribology of magnetic recording systems presents challenges well-known in the art. Although continual contact between the transducer (typically a read/write head mounted on a slider) and the magnetic medium is advantageous from a magnetic perspective, such arrangement is undesirable because wear and materials interactions lead to degraded system reliability and performance. Accordingly, one paramount consideration in designing magnetic recording systems is the maintenance of spacing between the head and medium.
The read/write head in a conventional recording system does not touch the rigid magnetic disk during sustained normal operation. Instead, it "flies" at a certain height (the "flying height") on a thin cushion of air carried along by the rapidly spinning disk. Typical flying heights are on the order of 12 microinches above the disk's surface.
The function of this cushion of air is analogous to that of a bearing, in that it corresponds to a machine part on which another part (the head) moves or slides with respect to the disk. In this regard, the air cushion supports the head in its position off of the disk.
Of course, a number of recording technologies do permit physical contact between head and storage medium during operation. Such technologies typically employ a flexible storage medium and are represented by tape and floppy disk recording systems. In these technologies, the concept and requirements of "flying height", and considerations relating to a bearing which functions to maintain flying height, are absent. This invention is not intended for use in such systems. Instead, this invention finds its dominant use in recording technologies in which a rigid, recording medium is moved (usually, rotated) with respect to a transducer, which must be maintained at a position off of ("above") the moving medium during system operation.
In the description following, the moving medium is assumed to be rigid, rotating magnetic disk, usually referred to as a "hard" or "Winchester" disk. However, the description of the embodiment is not intended to limit the scope of application of the invention, which could also apply, for example, to a spinning drum or, to a head in motion with respect to a stationary medium.
The disks in typical rigid-disk recording systems are commonly, but not always, coated with a relatively thin layer of lubricant, typically tens of Angstroms thick, to protect the head and disk surfaces during those occasional times when these surfaces touch, such as when the drive is jarred, or when the head passes over an asperity on the disk, or during take off and landing of the head. As is well known in the art, such lubrication is achieved by topical application.
These lubricating layers are always made with very viscous lubricants coated thinly onto the disk surfaces. This is done because less viscous lubricants would soon migrate to the edge of the disk due to centrifugal forces generated by the spinning disk and the lubricant layer would become so thin that it could no longer function effectively. Furthermore, less viscous lubricants would tend to have higher vapor pressures and tend to evaporate and further deplete the surface. If the lubricant is made thicker, then the head will be spaced further from the medium, which will degrade the performance of the system as is well known in the art.
There are several problems associated with conventional air bearings. First, air is relatively compressible. A small acceleration (as small as 40 g's) can exert a force on a head in a typical system great enough to cause the head to strike the disk, which, in spite of the presence of a lubricant, can lead to complete failure of the system. Such accelerations are easily caused by shocks and vibrations commonly encountered in the environment in which magnetic recording systems are employed. Second, when the distance from the head to the disk becomes comparable to the mean free path of air molecules, about three microinches, the air bearing becomes unstable and unreliable. As modern systems begin to approach this spacing limit, these instabilities in flying height can cause catastrophic contact between head and disk, and make it difficult for the system designer to know what record current to use when recording and what signal level to expect when reading information from the disk.
Present recording systems based upon use of a rigid disk experience the above-described shortcomings. Storage of information at even higher densities will require lower flying heights and smoother disks. Increased emphasis must, therefore, be placed on provision of a stable bearing operating between the head and the storage medium which supports the head over the medium at a distance which is smaller than that achievable by the air bearing of the prior art, yet which is mechanically simple and cost effective.
The prior art teaches the use of oil as a bearing material which can replace air. For example, Lynott's U.S. Pat. No. 2,969,435 describes an oil film spacer between a recording head and a rotating magnetic disk. In U.S. Pat. No. 3,005,675 of Ledin, et al., oil is used to form a fluid bearing between a magnetic head and a rotating drum having a magnetic recording surface. In Gabor's U.S. Pat. No. 3,579,212, a head and a storage medium are immersed in a Newtonian fluid while a related storage medium moves with respect to the head. The fluids used in these patents are Newtonian fluids which exhibit a constant viscosity. Furthermore, these references discuss head-surface spacings which are above even the spacing achievable with an air bearing. Because the described fluids are Newtonian, these prior art recording devices are relatively inefficient. This is because at very small head-surface spacings, a high speed induces a very high shear rates between the fluid and the head. As the shear rate increases, the drag force on the head increases commensurately, since the fluids are Newtonian. As the drag force increases, more power is required to rotate the disk.
In U.S. Pat. No. 4,633,351 of Bardos, et al., a hydrodynamic magnetic recording process is described in which a recording surface is moved relative to a recording head separated from the surface by a liquid of the type that will evaporate without leaving a residue. In this patent, all of the fluids proposed as evaporatible liquid bearings are Newtonian liquids, such as water, alcohol, and acetone.
In the magnetic recording arts, miniaturization of components is proceeding apace. Reduction of rigid disk standard size from 53/4" to 31/2" will put a premium on techniques which increase the storage density of the small disks. Of course, one extremely effective way of increasing storage density is to decrease the flying height of the recording head. However, reduction of the flying height raises the risks in air bearing mechanisms of head-disk contact. In mechanisms which use Newtonian fluid bearings, reduction of the head-surface spacing results in a significant increase in power required to rotate the disk. In portable systems, this translates to reduction of battery lifetime or to increase in the size and count of power elements.